Method of electrophotographically producing a multicolor image



H. G. GRElG Oct. 23, 1962 METHOD OF ELECTROPHOTOGRAPHICALLY PRODUCING A MULTICOLOR IMAGE 2 Sheets-Sheet 1 Original Filed March 20, 1958 INVENTOR. HARULD [5. E3215 Oct. 23, 1962 H. G. GRElG 3,060,020

METHOD OF ELECTROPHOTOGRAPHICALLY PRODUCING A MULTICOLOR IMAGE Original Filed March 20, 1958 2 Sheets-Sheet 2 INVENTOR. HAROLD 5.1331515 United States Patent C 5 Claims. (Cl. 961) This application is a division of my copending application Serial Number 722,707, filed March 20, 1958.

This invention relates to electrostatic printing and particularly, but not exclusively, to improved electroscopic developer powders for electrostatic printing and to improved methods of electrostatic printing utilizing the improved developer powders.

An electrostatic printing process is that type of process for producing a visible record, reproduction or copy which includes as an intermediate step, converting a light image or electrical signal into an electrostatic charge pattern on an electrically-insulating layer. The process usually includes the conversion of the charge pattern into a visible image which may be a substantially faithful reproduction of an original, except that it may be a different size.

A typical electrostatic printing process may include producing an over-all electrostatic charge on the surface of a photoconductive material such as selenium, anthracene or zinc oxide dispersed in an insulating binder. A light image is focused on the charged surface, discharging the portions irradiated by the light rays, while leaving the remainder of the surface in a charged condition, to thus form an electrostatic image. The electrostatic image is rendered visible by applying a developer powder which is held electrostatically to the charged areas of the surface. The powder image thus formed may be fixed directly to the photoconductive material or it may be transferred to another surface upon which the reproduced image may be desired and then fixed thereon. The fixing step commonly comprises fusing the developer powder to the photoconductive material by the application thereto of heat.

According to prior processes, reproduction of images in a plurality of colors could be accomplished by successively transferring powder images of different colored powders from the photoconductive surface to another surface. Briefly, this type of process comprises exposing a photoconductive plate, first, to an original through light filters which enable one color to 'be recorded, and then developing with colored powder to produce a copy of that color, then repeating for each other color and superimposing the powder images on the same copy sheet.

Production of plural color images by processes which require the transferring of each individual color image to a copy sheet introduces problems which are extremely diflicult to overcome. Registration of the separate images is probably the greatest of these problems. Elaborate precautions are necessary to insure that one image will be superimposed in exact registry with another upon a copy sheet. Other problems include: (a) loss of image detail and definition during transfer, (b) the surface of the photoconductive material must be cleaned after each image transfer, and (c) the photoconductive material is usually coated on one surface of a rigid plate making it diificult to transfer the powder image to non-flexible surfaces.

One object of this invention is to provide improved developer powders which facilitate electrostatic printing in a plurality of colors.

Another object of the invention is to provide improved developer powders for electrostatic printing and improved methods of electrostatic printing utilizing such developer powders.

Another object is to provide improved developer pow ders which make possible the electrostatic printing of plural color images in situ such that one color is caused to overlap another in desired areas.

A further object is to provide improved electroscopic developer powders and methods of electrostatic printing which obviate the need for any transfer steps in electrostatically producing plural color images.

A still further object is to provide an improved electrostatic printing process in which plural color images are printed in situ in a manner such that undesired overlapping of one color image by a subsequently produced color image is prevented.

In general, the foregoing objects and other advtntages may be accomplished in accordance with the instant invention which provides an improved electroscopic developer powder consisting essentially of finely divided particles of photoconductive zinc oxide coated with a film-forming material. The film-forming material is electroscopic, has a melting point substantially within the range of from 90 C. to 250 C. and a viscosity substantially within a range of from 45 to 10,000 centipoises at a temperature just above its melting point. The zinc oxide has a surface photoconductivity of at least about 10- ohm"- /square/watt/cm. and constitutes substantially from 50% to by weight of the developer powder. When desired, a suitable coloring agent may be incorporated in the coating.

The photoconductive zinc oxides herein described have the property of being able to hold an electrostatic charge in the dark or under safe-light conditions for a period at least long enough to complete an electrostatic printing process. When a developer powder made in accordance with this embodiment is fixed by fusing, the powder image so produced is capable of being electrostatically overprinted. Thus, a first powder image can be laid down on an insulating surface, such, for example, as photoconductive zinc oxide, and a second image of different color superimposed on top of the first. When suitable coloring agents are incorporated, colored developer powders are provided which can be superimposed one on the other to produce color copies which are substantially faithful reproductions of color prints.

The developer powders of this invention possess another useful property. These powders are capable of being processed in accordance with a procedure, to be described hereinafter, which renders them incapable of receiving a superimposed powder image. Thus, the developer powders of this invention are also useful in electrostatic printing processes for producing plural color images in contiguous areas on an insulating surface.

In accordance with this invention improved electrostatic printing methods are provided utilizing the above described electroscopic developer powders. One such method comprises the steps of 1) developing a latent electrostatic image on an insulating surface by applying thereto an electroscopic developer powder of the type described heretofore; (2) applying heat to said developer powder to cause the coating on the zinc oxide particles to melt and flow toward the insulating surface thereby causing portions of the particles of zinc oxide to protrude above the molten coating material and leaving on those portions only a very thin film of coating material; (3) producing a second electrostatic image on the insulating surface having thereon the first developed image; (4) applying a different electroscopic developer powder to the second electrostatic image to produce a second powder image; and (5) fixing the second powder image. When desired, the steps of the above method may be '2 repeated to produce, in as many colors as desired, a composite color image.

With a slight modification of the foregoing method electrostatic printing of plural images in contiguous areas on an insulating surface is made possible. All that is required to produce this result is the inclusion of an additional step between steps (2) and (3) described above. This step comprises uniformly flooding with light the insulating surface bearing the first developed image thereon. When this step is employed, superimposition of one developed image upon another is prevented and production of plural images in contiguous areas results.

Other objects and advantages of this invention are more fully described in the following detailed description when read in conjunction with the accompanying drawings wherein:

FIG. 1 is a partially-schematic sectional view of an apparatus for producing a blanket electrostatic charge upon an insulating surface.

FIG. 2 is a partially-sectional elevational view of an apparatus for projecting light to form by contact an electrostatic image upon the insulating surface of FIG. 1.

FIG. 3 is a sectional view of an apparatus for applying electroscopic developer powder, in accordance with this invention, to the image produced in FIG. 2..

FIG. 4 is a partially-schematic sectional view of an apparatus for fixing the developed image, produced in FIG. 3, to the insulating surface.

FIG. 5 is a partially-schematic sectional view illustrating the result obtained with the apparatus of FIG. 3 in accordance with the method of this invention.

FIG. 6 is similar to FIG. 5 but illustrates the result obtained in accordance with a modification of the method of this invention.

. Similar reference characters are applied to similar elements throughout the drawings.

ZINC OXIDES The selection of a Zinc oxide which is a good photoconductor for electrostatic printing is an important feature of this invention. Several methods have been devised to distinguish between those which are suitable and those which function poorly or not at all in electrostatic printing.

Method 1.A mixture is prepared comprising about milligrams of dry zinc oxide powder and a few drops of an 80% solution of silicone resin in xylene (G.E.-SR 82, marketed by the General Electric Company, Silicone Products Division, Waterford, N.Y.) diluted with toluene in the ratio 60 grams solution to -105 grams toluene. The mixture is coated on filter paper and dried to produce a dry coating over an area about 0.25 inch in diameter. The dry coating is cooled to about 190 C. and examined in light from a mercury vapor lamp having a maximum output at about 3650* A. The zinc oxides which produce printable coatings produce a lavender or orange luminescence. Other Zinc oxides exhibit a green or yellow luminescence.

Method 2.About 0.25 gram of dry zinc oxide powder is placed in a silica boat. The boat is inserted into a silica tube and the system flushed with hydrogen gas. The tube and boat are fired for about 5 minutes at about 1000 C. in a stagnant hydrogen atmosphere. The boat is cooled in hydrogen to room temperature. The fired zinc oxide is examined in light from a mercury vapor lamp having a maximum output at about 3650 A. The zinc oxides which produce printable coatings luminesce brightly. Other zinc oxides luminesce weakly or not at all.

A preferred photoconductive zinc oxide is one which produces a lavender color in Method 1 and luminesces brightly in Method 2.

It has been found that the useful Zinc oxides selected in accordance with the above procedures have a surface photoconductivity of at least about 10 ohm /square/ watt/cm. when subjected to light of a wavelength of about 3900 A. The surface photoconductivity of a zinc oxide can be determined by applying thereto a third method, as follows:

Method 3.-A small quantity of Zinc oxide is reduced to a powder and compressed under high pressure (about 15,000 lbs. per square inch) to form a pellet. Electrodes, as of silver paste, are applied on the surface of the pellet leaving a square area of surface uncoated. The pellet is then placed in a monochromator with the aforementioned uncoated surface area facing the light source and successive wavelengths of light throughout the spectrum are projected on this surface. The light beam projected onto the surface is chopped at about 23.5 c.p.s. by a constant speed rotating disc, pierced to produce equal intervals of light and darkness. A D.-C. potential is placed across the electrodes and the current flowing between the electrodes is measured as a function of wavelength with the intensity of radiation being held constant.

The zinc oxides which are suitable are those which are substantially electrically non-conductive in the dark. When exposed to light, they should exhibit a surface photoconductivity of a certain level in order to be of practical use for the purposes of this invention. In testing zinc oxides to determine their suitability and utilizing a pellet form, it is convenient to express the results of the measurements of the test as surface photoconductivity because substantially all of the light is absorbed in a thin layer at the surface of the pellet. It has been found that, to be useful in this invention, the zinc oxide selected should have a surface photoconductivity of at least 10- ohm /square/watt/crn. when exposed to a wavelength of about 3900 A.

The foregoing methods show that the class of zinc oxides, known as French process Zinc oxides, generally function as good photoconductors and that the class known as American process zinc oxides generally function as nonphotoconductors for the purposes of electrostatic printing.

ZINC OXIDE COATINGS Proper selection of a suitable coating material for the particles of zinc oxide is another important feature of this invention. A material is normally selected having a melting point less than the temperature at which paper will char. A preferred temperature range is between C. and 250 C. It is also important that the coating be one which, when applied to an insulating surface and fused thereon, will function as a binder holding the zinc oxide on the insulating surface. Once the developer powder is fixed or fused to the insulating surface it must itself have insulating properties. Thus, a preferred coating material has a high dielectric strength. The viscosity of the coating material comprises another important criterion. The viscosity must be low enough so that when melted the coating will flow off the particles of zinc oxide leaving them partially exposed or protruding with only a very thin film remaining on the exposed or protruding portions of zinc oxide particles. After the coating material has been melted and fused to an insulating surface it is essential that the protruding particles of zinc oxide shall present a matte surface i.e. the film of coating material remaining on the protruding portions of zinc oxide must not be thick enough to provide a gloss finish which would impair subsequent printing operations. It is extremely difficult to measure the thickness of such a film, however, if the physical apearance substantially like that described, is achieved the developer powder will have the characteristics contemplated in this invention. it is preferred that the coating should not be so free flowing as to allow it to migrate into unwanted areas of the insulating surface when melted. A preferred viscosity range is from 45 cps. to 10,000 cps. as measured with a direct reading Brookfield viscosimeter with a spindle speed of 60 rpm.

at a temperature just slightly above the melting point of the material. Finally, it is essential that the coating on the particles of zinc oxide have electroscopic properties so that the coated particles may be electrostatically attracted to charged areas of the insulating surface.

Coating materials having the foregoing properties may comprise certain natural or synthetic resins, waxes or other low melting materials or mixtures thereof. For example any of the following materials or combinations of materials may be used:

(1) Carnauba wax.

(2) Polymekon Wax (a chemically modified microcrystalline wax of the Warwick Wax Co., New York, N.Y.).

(3) Ultracera Amber Wax (a microcrystalline petroleum wax of the Bareco Oil Co., Barnsdall, Oklahoma).

(4) BE Square Wax White (a microcrystalline petroleum wax of the Bareco Oil Co.).

(5) Petronauba D Wax (a microcrystalline petroleum wax of the Bareco Oil Co.).

(6) Piccolyte S135 (a thermoplastic hydrocarbon terpene resin of the Pennsylvania Industrial Chemical Corp., Clairton, Pennsylvania).

(7) A mixture of Polymekon Wax and Piccolyte 8-115.

(8) A mixture of Acrawax C (a synthetic wax-octa-' decenamide, of the Glyco Products Co., Brooklyn, New York) and calcium stearate.

(9) A mixture of Acrawax C and a solid silicone resin.

Coating materials such as those specified may also include modifying agents such as plasticizers, toughening agents, hardening agents, dispersing agents, etc., which are added to obtain desired physical and electrical properties.

A developer powder of this invention includes a ratio of zinc oxide to coating material within a range of from 1 to 7 parts by weight of zinc oxide to 1 part by weight of coating material. Such a powder is generally prepared by first melting the coating material and then dispersing finely divided zinc oXide in the melt. The melt is allowed to cool and harden after which it is broken up and reduced to the desired powder form. The ratio of zinc oxide to coating material specified above is important in a given developer powder formula. The exact ratio depends to a large extent on the particle size and the dispersion of the zinc oxide chosen.

COLORING AGENTS Coloring agents such as dyes, stains or pigments can be added to the melt to produce developer powders of a desired color. Examples of suitable coloring agents include:

1) Cyan Blue Toner GT (describedin US. Patent 2,486,351 to Richard H. Wiswall, Jr.).

(2) Benzidine Yellow.

(3) Brilliant Oil Blue BMA (Color Index No. 0.1. 61555, National Aniline Division of Allied Chemical and Dye Corp., New York, N.Y.).

(4) Sudan IH Red (Color Index No. 2610 0, Fisher Scientific Co., Pittsburgh, Pennsylvania).

(5) Oil Yellow 2 G (Color Index No. 11020, American Cyanamid, New York, N.Y.).

These and other suitable coloring agents may be employed singly or in combination to impart to the developer powder a desired color.

SENSITIZING AGENTS In addition to the foregoing, various sensitizing agents may be employed to vary the spectral response of a photoconductive zinc oxide. For example: photoconductive white zinc oxide has a spectral sensitivity having a peak in the ultraviolet range. By adding various sensitizing agents, a white photoconductive zinc oxide can be provided having an additional sensitivity peak in other por- 6 tions of'the spectrum. Satisfactory sensitizing agents include the following:

(l)Fluorescein Sodium (Color Index No. 766). (2) Eosin Y (Color Index No. 768).

(3) Rose Bengal (Color Index No. 779).

(4) Brilliant Green (Color Index No. 662).

(5) Patent Blue (Color Index No. 672).

( 6) Thiofiavin TG (Color Index No. 49005).

PRINTING PROCESSES The improved methods in accordance with this invention will now be described with reference to the drawings.

FIG. 1 illustrates a means for applying a uniform electrostatic charge to an insulating surface 11. Usually, in electrostatic printing, the insulating surface 11 will comprise a photoconductive coating on a substrate 12 which is illustrated herein as a sheet of paper. The sheet 12 is positioned on a grounded metal plate 13 following which a corona charging unit 14 is passed one or more times over the insulating surface to provide thereon a uniform electrostatic charge.

The next step in the process, illustrated in FIG. 2, is to produce an electrostatic image on the photoconductive insulating surface 11. This may be accomplished by placing a photographic transparency 21 upon the charged photoconductive insulating surface 11 and exposing to light derived, for example, from a lamp 22 in the manner of conventional contact printing. Wherever the light strikes the surface 11, the electrostatic charge thereon is reduced or removed. This leaves an electrostatic image or pattern of charges corresponding to the non-illuminated areas of the light image.

The electrostatic image may be stored for a while if desired. Ordinarily, the next step is to apply developer powder to the surface 11. Referring to FIG. 3 this may be accomplished by passing a developer brush 31 containing the developer powder across the surface 11 bearing the electrostatic image. Coated particles 32 of developer powder are deposited on those areas of the surface 11 retaining the electrostatic charge. The developer brush comprises a mixture of magnetic carrier particles, for example powdered iron, and the developer powder. The mixture is secured in a magnetic field by a magnet 33 to form the developer brush 31.

Methods of charging, exposing and developing may be employed other than those described with reference to FIGS. 1, 2, and 3. For example, charging may be accomplished by friction, exposure by projection, and development by dusting the developer powder onto the insulating surface, all as well known in the art.

For a more detailed description of the corona charging method of FIG. 1 and the magnetic brush development of FIG. 3, reference is made to ElectrofaxDirect Electrophotographic Printing on Paper, by C. J. Young and H. G. Greig, RCA Review, volume 15, No. 4. Also described in this publication is a method of development called cascading. Cascading utilizes gravity to convey the developer powder, mixed with a carrier such as glass beads, across the insulating surface. This method of development is also contemplated in this invention.

The developed image is now fixed to the surface. This is easily accomplished, as shown in FIG. 4, by passing a resistance heating unit 41 over the image-bearing photoconductor insulating surface 11. When a temperature above the melting point of the coating on the zinc oxide particles is applied thereto, the coating melts and becomes bonded to the surface 11. Other means are available for fusing the developed image. For example, the heating element 41 may comprise an infrared lamp, or the sheet 12 may be placed in an oven.

It is during the fixing step that one of the unique features of this invention becomes apparent. When coating materials for the zinc oxide particles are selected as taught herein, a surface is obtained in developed areas as shown in FIG. 4. The coating material melts to form a continuous layer 42 adhering to the surface 11. In forming this continuous layer 42 the coating material melts off portions of at least the topmost particles of zinc Oxide 43 leaving these portions protruding above the layer 42 and also leaving only a very thin film of coating material on the protruding portions of the particles. Were the coating material to have too high a viscosity, for example greater than 10,000 cps, it is very unlikely that this result could be achieved. Such coating material would tend to adhere to the entire surface of the particles of Zinc oxide even when heat is applied. Thus the zinc oxide particles would tend to be covered with a layer of insulating material too thick to permit discharge by exposure to light. Were the coating material to have too low a viscosity, for example less than cps., there would be a tendency for it to spread into non-image areas thereby causing a loss in definition of the image. Because of this unique characteristic of the coating material a novel electrostatic printing process is possible as will be described hereinafter.

OVERPRINTING This process involves the overprinting of a first powder image with a second. This is made possible by employing in the development step, described in connection with FIG. 3, coated particles of photoconductive zinc oxide. Upon completion of the fixing step of FIG. 4, these particles provide surfaces in the image areas of the photoconductive insulating surface 11 which are capable of retaining an electrostatic charge in the dark or under safe light conditions. Accordingly, subsequent to the fixing step of FIG. 4, this first proces comprises the steps of (1) recharging the image bearing surface 11 with an overall charge as shown in FIG. 1, (2) exposing the surface to another light image as shown in FIG. 2 to produce a second electrostatic image and (3) developing the second electrostatic image with a different developer powder. The results of this process are shown in FIG. 5. As shown therein the developer powder first deposited consists of zinc oxide particles 43' and the fused coating material 42. Coated zinc oxide particles 32' are deposited in configuration with the second electrostatic image. Because the zinc oxide in the first developer powder was photoconductive, the second image overlaps the first in those areas upon which light does not impinge during the second exposure, i.e., overprinting of the first image occurs in all areas where the second electrostatic image is superimposed upon the first developed image. Of course, it is also evident that any portions of the areas of the insulating surface 11 not covered by the first developer powder may also become image areas during the second exposure. Developer powder will also be attracted to these areas to complete the second powder image. Charging, exposure, developing and fixing can be carried out a third time to produce a third overprinted powder image. The last development step does not require the use of a photoconductive zinc oxide but, instead, may be accomplished with any type of developed powder commonly employed in the art of electrostatic printing.

In accordance with this process, printing in natural colors is made possible. For example: light may be projected through a color transparency and then through filters capable of transmitting all colors except yellow. The electrostatic image so produced is then developed with yellow colored developer powder. A second exposure is made through filters transmitting all colors except blue and the development carried out with blue colored developer powder. In the same way, red light is filtered out and red developer powder overprinted over the yellow and blue. Thus, in a three stage process, yellow, blue and red powder images are superimposed on one another to provide a composite image in natural colors.

NON-OVERPRINTING A different result made possible by the unique characteristics of the coating material comprises producing a powder developed image which cannot be overprinted in subsequent electrostatic printing steps. This is accomplished by uniformly flooding the image bearing insulating surface with light after the fusing step of FIG. 4 but before repeating the charging step of FIG. 5. This is easily done by exposing the surface 11 to the lamp 22 of FIG. 2 with the photographic transparency 21 removed to uniformly flood the surface with light. When this step is employed, a fused powder image is rendered incapable of retaining an electrostatic charge when the charging step of FIG. 1 is repeated. Thus, when a two-stage procedure similar to that described with respect to overprinting is carried out, the results obtained are as shown in PEG. 6. The developer powder first deposited consists of zinc oxide particles 43" and the fused coating material 42. The second developer powder consisting of coated zinc oxide particles 32 is deposited in configuration with the second electrostatic image. However, since the zinc oxide particles 43 have been rendered incapable of retaining an electrostatic charge, deposition of the second developer powder will only occur in those areas on the surface 11 not covered by the first deposited developer powder. Therefore, in accordance with this modification, composite color images can be produced consisting of two, three or more colors deposited in contiguous areas on an insulating surface.

PHOTOCONDUCTIVE DEVELOPER POWDERS The following group of examples provide developer powders suitable for use in the process described above. These powders, when fused, present a surface which can be overprinted in subsequent electrostatic printing steps.

Example I \VI-HTE DEVELOPER POWDER Parts by weight Carnauba wax 1 Photoconductive French process Zinc oxide 2 This is the simplest type of overprinting developer powder. The wax is melted and particles of the zinc oxide having a particle size of from .025 to .5 micron mean diameter are added to the melt. Particle size and shape of the zinc oxide determine to some extent the ratio of zinc oxide to coating material. Due to the bulking characteristic of zinc oxide finer particles usually require more coating material since there is more total surface to be covered. Continuous stirring from 15 to 30 minutes is sufficient to thoroughly disperse the zinc oxide in the wax when the batch weighs about grams. The mixture is then allowed to cool and harden after which it is reduced to a fine powder. This is accomplished by ball milling the mixture for about 3 hours and then classifying it as to particle size. For most purposes, the fraction below 200 mesh (74 microns) is suitable for use as an electroscopic developer powder.

Example II WHITE DEVELOPER POWDER Parts by weight Acrawax C 20 Photoconductive French process zinc oxide 30 Preparation the same as in Example I.

Example Ill BLUE DEVELOPER POWDER Parts by weigh Acrawax C 20 Photoconductive French process zinc oxide 30 Calcium stearate (pigment wetting agent) 0.3 Cyan Blue Toner G.T 1.5

Preparation same as in Example 1 except that the cal- 9 cium stearate is added to the melt before the zinc oxide, and the coloring agent after the zinc oxide.

Example IV BLUE DEVELOPER POWDER Parts by weight Acrawax C 36 Silicone resin (solid) (Dow Corning No. R5071) 4 Photoconductive French process zinc oxide 70 Cyan Blue Toner G.T 3

Preparation same as in Example I except the Acrawax C and silicon resin are melted together before adding the zinc oxide and coloring agent.

Preparation as in Example IV.

Example VI RED DEVELOPER POWDER Parts by weight Acrawax C 3o Silicone resin (solid) (Dow Corning No. R5071) 5 Photoconductive French process zinc oxide 80 Benzidine Yellow 3 Preparation as in Example IV.

Example VII GREEN DEVELGPER POWDER Parts by weight Piccolyte S-l35 20 Photoconductive French process zinc oxide 30 Benzidine Yellow 1 Brilliant Oil Blue B.M.A 0.23

Preparation as in Example I except add the coloring agents after adding the zinc oxide to the molten Piccolyte.

Example VIII GREEN DEVELOPER PO'WDER Parts by weight Piccolyte S-135 20 Photoconductive French process zinc oxide 30 Cyan Blue Toner G.T 1 Benzidine Yellow 1 Preparation as in Example VII.

SENSITIZING AGENTS The following sensitizing agents may be employed with any of the foregoing examples of overprinting developer powders. These sensitizing agents are added to the melt after the zinc oxide and coloring agents have been added.

Example IX Add Patent Blue to Examples I to VIII in a ratio of about 0.05 part Patent Blue to 70 parts by weight of zinc oxide. This provides a photoconductive developer powder having a second sensitivity peak of about 6380 A. in the orange portion of the spectrum.

Example XI Add Thioflavin T.G. to Examples I to vVIII in a ratio of about 0.1 part of Thiofiavin to about 70 parts by weight of zinc oxide. This provides a photoconductive developer powder having a second sensitivity peak at about 4080 A. in the blue portion of the spectrum.

Example XII Add Rose Bengal to Examples I to VIII in a ratio of about 0.05 part of Rose Bengal to about 70 parts by Weight of zinc oxide. This provides a photoconductive developer powder having a second sensitivity peak at about 5500 A. in the green portion of the spectrum.

Example XIII Add Fluorescein Sodium to Examples I to VIII in a ratio of about 0.05 part of Florescein to about 70 parts by Weight of zinc oxide. This provides a photoconductive developer powder having a second sensitivity peak at about 4770 A. in the blue-green portion of the spectrum.

Example XIV Add Eosin Y to Examples I to VIII in a ratio of about 0.05 part of Eosin Y to 70 parts by weight of zinc oxide. This provides a photoconductive developer powder having a second sensitivity peak at about 5000 A. in the green portion of the spectrum.

When French process White zinc oxide is employed in the developer powder without any sensitizing agent added thereto the powder has a single sensitivity peak at about 3750 A. in the ultraviolet portion of the spectrum. When one of the dyes of Examples IX-XIV is added, the developer powder still exhibits a peak at about 3750 A. as well as an additional peak at the wavelength cited for each example. Thus, there is provided means for ob-' taining a spectral response at almost any portion of the spectrum from ultraviolet to red.

There have been described new and improved electroscopic developer powders and methods of elestrostatic printing which make possible electrostatic printing in a plurality of colors.

What is claimed is:

1. The method of electrostatic printing comprising the steps of electrophotographically producing a first electrostatic image on a photo-conductive insulating surface, applying to said electrostatic image a first developer powder having a characteristic color and consisting essentially of particles of photoconductive zinc oxide having a surface photoconductivity of at least 10 ohm /square/watt/ cm. at a wavelength of about 3900 A., said particles having a coating thereon of a thermoplastic, electroscopic, insulating material having a melting point substantially within a range of from C. to 250 C. and a viscosity substantially within a range of from 45 to 10,000 centipoises at a temperature slightly above the melting point of said material, applying heat to said developer powder on said insulating surface to cause said coating to melt and flow toward said said insulating surface leaving said particles of zinc oxide protruding from said coating material and leaving only a thin film of coating material on the portions of said particles protruding from said coating material to provide in image areas on said insulating surface a photoconductive insulating coating, electrophotographically producing a second electrostatic image on said insulating surface overlapping said photoconductive insulating coating, and applying to said second electrostatic image a second electroscopic developer powder having a characteristic color different from that of said first developer powder.

2. The method of electrostatic printing comprising the steps of electrophotographically producing a first electrostatic image on an insulating surface, applying to said electrostatic image a first developer powder having a characteristic color and consisting essentially of particles of photoconductive zinc oxide having a surface photoconductivity of at least 10" hm /square/watt/cm. at a wavelength of about 3900 A., said particles having a coating thereon of a thermoplastic, electroscopic, insulating material having a melting point substantially within a range of from about 90 C. to 250 C. and a viscosity substantially within a range of from 45 to 10,000 centipoises at a temperature slightly above the melting point of said material, applying heat to said first developer powder on said insulating surface to cause said coating to melt and flow toward said insulating surface leaving particles of said zinc oxide protruding from said coating material and leaving only a thin film of coating material on the portions of said particles of zinc oxide protruding from said coating material to provide in image areas on said insulating surface a photoconductive insulating coating, electrophotographically producing a second electrostatic image on said insulating surface overlapping said photoconductive insulating coating, applying to said second electrostatic image a second developer powder substantially the same as said first developer powder but having a characteristic color differing therefrom, applying heat to said insulating surface bearing said photoconductive insulating coating and said second de- .veloper powders to melt the coating on the particles of zinc oxide of said second developer powder to cause said coating to melt and flow toward said insulating surface leaving particles of said Zinc oxide of both said photoconductive insulating coating and said second developer powders protruding from said coating materials leaving only a thin film of coating material on the portions of said particles of zinc oxide protruding from said coating material to provide on said insulating surface additional photoconductive insulating coating, electrophotographical- 1y producing a third electrostatic image on said insulating surface overlapping said photoconductive insulating coatingand said additional photoconductive coating, and applying a third developer powder having a characteristic color different from either said first or said second developer powders to said third electrostatic image.

3. The method of electrostatic printing comprising the steps of producing a substantially uniform electrostatic charge upon a photoconductive insulating surface, exposing said photoconductive surface to a light image to produce thereon a first electrostatic image, applying to said first electrostatic image a first developer powder having a first characteristic color and consisting essentially of particles of photoconductive Zinc oxide having a surface photoconductivity of at least ohm /square/watt/ cm. at a wavelength of about 3900 A., said particles having a coating thereon of a thermoplastic, electroscopic, insulating material having a melting point substantially within a range of from 90 C. to 250 C. and a viscosity substantially within a range of from 45 to 10,000 centipoises at a temperature slightly above the melting point of said material, applying heat to said first developer powder on said photoconductive surface to cause said coating to melt and how toward said surface leaving said zinc oxide protruding from said coating material and leaving only a thin film of coating material on the portions of said particles of zinc oxide protruding from said coating material to provide in image areas on said insulating surface a photoconductive insulating coating, producing a substantially uniform electrostatic charge upon said photoconductive surface with said photoconductive insulating coating thereon, exposing said photoconductive coating with said photoconductive insulating coating thereon to a second light image to produce a second electrostatic image in areas on said surface covered by said photoconductive insulating coating as well as in areas not so covered, and applying a second developer powder having a characteristic color different from said first developer powder to said second electrostatic image.

4. The method of electrostatic printing comprising the steps of producing a substantially uniform electrostatic charge on a photoconductive insulating surface, exposing said photoconductive surface to a first light image to produce thereon a first electrostatic image, applying to said electrostatic image a yellow developer powder consisting essentially of particles of photoconductive Zinc oxide having a surface photoconductivity at least 10- ohm- /square/watt/cm. at a wavelength of about 3900 A., said particles having a coating thereon of a thermoplastic, electroscopic, insulating material including a minor proportion of a yellow coloring agent, said material having a melting point substantially within a range of from C. to 250 C. and a viscosity substantially within a range of from 45 to 10,000 centipoises, applying heat to said developer powder on said photoconductive surface to cause said coating to melt and flow toward said surface leaving said particles of zinc oxide protruding from said coating material and leaving only a thin film of coating material to produce in image areas on said surface on the portions of said particles of zinc oxide protruding from said coating a yellow photoconductive insulating coating producing a substantially uniform electrostatic charge upon said photoconductive surface with said yellow coating thereon, exposing said photoconductive surface with said yellow coating thereon to a second light image overlapping areas on said surface covered by said yellow coating to produce a second electrostatic image, applying to said second electrostatic image a blue developer powder consisting essentially of particles of photoconductive zinc oxide and a coating material substantially the same as the Zinc oxide and coating material of said first developer powder and including a minor proportion of a blue coloring agent, applying heat to said photoconductive surface bearing said yellow coating and said blue developer powder to cause the coating material of said blue developer powder to melt and flow toward said surface leaving the particles of zinc oxide of both said yellow coating and said blue developer powder protruding above said coating material and leaving only a thin film of coating material on the portions of all of said protruding particles of Zinc oxide to provide a combined yellow and blue photoconductive insulating coating on said surface, producing a substantially uniform electrostatic charge upon said photoconductive surface with said combined coating thereon, exposing said photoconductive surface bearing said combined coating thereon to a third light image overlapping areas on said surface covered by said combined coating to produce thereon a third electrostatic image, applying to said third electrostatic image a red developer powder and fixing said red developer powder on said surface.

5. The method of electrostatic printing comprising the steps of: (l) producing a substantially uniform electrostatic charge upon a photoconductive insulating surface; (2) exposing said photoconductive surface to a first light image to produce therein a first electrostatic image; (3) developing said first electrostatic image by applying thereto a yellow developer powder consisting essentially of particles of photoconductive zinc oxide having a surface photoconductivity of at least l0 ohm /square/watt/ cm. at a wavelength of about 3900 A., said particles having a coating thereon of a thermoplastic, electroscopic, insulating material including a minor proportion of a yellow coloring agent, said material having a melting point substantially within a range of from 90 C. to 250 C. and a viscosity substantially within a range of from 45 to 10,000 centipoises; (4) fixing said developer powder to said photoconductive surface by applying heat thereto to cause said coating to melt and flow toward said surface leaving said particles of Zinc oxide protruding from said coating material and leaving only a thin film of coating material on the portions of said particles protruding from said coating material to provide on said surface a yellow photoconductive insulating coating; (5) repeating the procedure of steps (1), (2), (3) and (4) employing therein a blue developer powder consisting essentially of a Zinc oxide and a coating material substantially the same as the zinc oxide and coating material of said yellow developer powder and including a minor proportion of a blue coloring agent to produce thereby a combined yellow and blue photoconductive insulating coating affixed to said photoconductive surface, the yellow and blue components of said combined coating overlapping in at least some areas on said surface to produce color mixing; (6) repeating the procedure of steps (1), (2), (3) and (4) employing therein a red developer powder consisting essentially of a zinc oxide and a coating material substantially the same as the zinc oxide and coating material of said yellow developer powder and including therein a minor proportion of a red coloring agent to produce thereby a combined yellow, blue and red photoconductive insulating coating aflixed to said photoconductive surface the yellow, blue and red components of said last named combined coating overlapping in at least some areas on said surface to produce color mixing; (7) repeating the procedure of steps (1), (2), and (3) employing therein References Cited in the file of this patent UNITED STATES PATENTS 2,297,691 Carlson Oct. 6, 1942 2,584,695 Good Feb. 5, 1952 2,735,785 Greig Feb. 21, 1956 2,758,939 Sugarman Aug. 14, 1956 2,808,328 Jacob Oct. 1, 1957 2,868,642 Hayford et al. Jan. 13, 1959 2,894,840 Dessauer et al. July 14, 1959 2,924,519 Bertelsen Feb. 9, 1960 2,972,304 Jarvis Feb. 21, 1961 OTHER REFERENCES Rydz et al.: R.'C.A. Review, 19, 465-86 (1956). 

1. THE METHOD OF ELECTROSTATIC PRINTING COMPRISING THE STEPS OF ELECTROPHOTOGRAPHICALLY PRODUCING A FIRST ELECTROSTATIC IMAGE ON A PHOTO-CONDUCTIVE INSULATING SURFACE, APPLYING TO SAID ELECTROSTATIC IMAGE A FIRST DEVELOPER POWDER HAVING A CHARACTERISTIC COLOR AND CONSISTING ESSENTIALLY OF PARTICLES OF PHOTOCONDUCTIVE ZINC OXIDE HAVING A SURFACE PHOTOCONDUCTIVITY OF AT LEAST 10-9 OHM-1/SQUARE/WATT/ CM.2 AT A WAVELENGTH OF ABOUT 3900 A., SAID PARTICLES HAVING A COATING THEREON OF A THERMOPLASTIC, ELECTROSCOPIC, INSULATING MATERIAL HAVING A MELTING POINT SUBSTANTIALLY WITHIN A RANGE OF FROM 90*C. TO 250*C. AND A VISCOSITY SUBSTANTIALLY WITHIN A RANGE OF FROM 45 TO 10,000 CENTIPOISES AT A TEMPERTURE SLIGHTLY ABOVE THE MELTING POINT OF SAID MATERIAL, APPLYING HEAT TO SAID DEVELOPER POWDER ON SAID INSULATING SURFACE TO CAUSE SAID COATING TO MELT AND FLOW TOWARD SAID SAID INSULATING SURFACE LEAVING SAID PARTICLES OF ZINC OXIDE PROTRUDING FROM SAID COATING MATERIAL AND LEAVING ONLY A THIN FILM OF COATING MATERIAL ON THE PORITONS OF SAID PARTICLES PROTRUDING FROM SAID COATING MATERIAL TO PROVIDE IN IMAGE AREAS ON SAID INSULATING SURFACE A PHOTOCONDUCTIVE INSULATING COATING, ELECTROPHOTOGRAPHICALLY PRODUCING A SECOND ELECTROSTATIC IMAGE ON SAID INSULATING SURFACE OVERLAPPING SAID PHOTOCONDUCTIVE INSULATING COATING, AND APPLYING TO SAID SECONE ELECTROSTATIC IMAGE A SECOND ELECTROSCOPIC DEVELOPER POWDER HAVING A CHARACTERISTIC COLOR DIFFERENT FROM THAT TO SAID FIRST DEVELOPER POWDER. 